The present invention relates to threshold circuits for comparators. More particularly, the present invention relates to novel adaptive threshold circuits for generating the threshold voltage for comparators that compare an input signal to a threshold voltage, whereby the generated threshold voltage adapts to characteristics of the input signal. Even more particularly, the present invention relates to adaptive threshold circuits capable of adapting the threshold voltage to the output of a photo diode preamplifier.
In many digital communication systems, a photo diode receiver is used to detect lightwave pulses transmitted by a light emitting diode ("LED"). The photo diode typically provides an input current to a preamplifier, which converts the input current to voltage and adds modest gain. A comparator reconstructs the received pulses by comparing the preamplifier output voltage with a threshold voltage. Such photo diode receivers typically operate over a wide range of frequencies, pulse amplitudes, and ambient light conditions.
Because a photo diode's current is proportional to incident light, ambient light can cause large DC offsets and unwanted low frequency AC signals at the preamplifier output. To compensate for such undesired effects, photo diode receivers typically employ a transconductance block in conjunction with the preamplifier, forming a servo loop. The transconductance block feeds a current back into the preamplifier input so that the preamplifier output equals a preset bias level when no lightwave pulses are received.
To reconstruct received pulses, the comparator's negative input is coupled to the preamplifier output, and the comparator's positive input is connected to a threshold voltage that is set to a level half-way between the preset bias level and some predetermined minimum pulse amplitude. The comparator's output changes from HIGH-to-LOW whenever the preamplifier output voltage exceeds the threshold voltage, and changes from LOW-to-HIGH whenever the preamplifier output voltage falls below the threshold voltage. In this configuration, any DC offset at the preamplifier output appears as a differential-mode offset at the comparator's input terminals. Ideally, the servo loop cancels out any DC and low frequency components of the photo diode output signal such that the comparator passes only pulses above the threshold voltage.
The servo loop does not, however, cancel DC offset generated in the transconductance block. The transconductance block typically has an input stage including a highly degenerated differential amplifier that has wide dynamic range, but also has a large input-referred DC offset. This offset appears unattenuated at the preamplifier output and may be of the same order of magnitude as the amplified photo diode pulses. If the photo diode receiver must capture low level signals, this DC offset at the preamplifier output may be greater than the threshold voltage. As a result, the comparator may not capture any pulses because the preamplifier's output signal will likely never cross the threshold voltage. Typically, a trim circuit reduces such offset errors by adding a compensating offset to the transconductance block input. A problem with this approach, however, is that each circuit individually must be adjusted, or "trimmed," thus increasing time and expense during the manufacturing process.
Furthermore, typical "fixed threshold with trim" circuits often cannot adapt to certain unique characteristics of photo diodes. At high pulse amplitudes, the photo diode conducts a non-zero current for a period of time after the received light pulse has ended. As a result of this current, the preamplifier output exhibits a small sharp drop at the end of the pulse (the "sharp cutoff region") followed by a relatively slower decay (the "tail region") down to a DC level. If the photo diode receiver operates over a wide frequency range and pulse amplitude range, it is possible that the tail region from one pulse will not decay below the preset threshold voltage before the next pulse is received. If this occurs, the comparator may once again fail to properly detect received pulses. Further, even if the tail region decays below the threshold voltage, the reconstructed pulse width will be longer than the received lightwave pulse width.
One way to counteract these DC offset errors is to replace the "fixed threshold with trim" circuit with a circuit that provides an adaptive trigger threshold, such as the Fast Single Supply Adaptive Trigger circuit shown and described in FIG. 3 at page 10-13 in the 1992 Linear Databook published by Linear Technology Corporation of Milpitas, California. That adaptive trigger circuit sets the comparator threshold voltage at the midpoint of the input signal's highest and lowest peak amplitudes. The circuit uses two peak detectors to capture on two capacitors the input signal's high peak and low peak (lowest level). A pair of high-valued series-connected resistors are connected to the two capacitors and are used to (1) set the discharge time for the high peak capacitor and the charge time for the low peak capacitor, and (2) form a voltage divider that sets the threshold voltage between the voltages on the two capacitors. The adaptive trigger circuit removes the need for a trim circuit because the threshold voltage is not set to a fixed level, but adapts to the signal with which it is being compared.
One drawback of the foregoing adaptive trigger circuit, however, is that it requires an initial peak to set the threshold voltage. Because the initial threshold voltage equals the steady state DC input signal amplitude, the comparator's response to the first pulse is unpredictable. Further, the foregoing adaptive trigger circuit requires large valued resistors to maintain a large discharge time for the high peak capacitor and a large charge time for the low peak capacitor, so that the threshold voltage remains for an adequate duration at a voltage midway between the input signal's high and low peaks.
Moreover, if the foregoing adaptive trigger circuit receives no lightwave pulses during a steady-state period and then suddenly receives large valued photo diode pulses, the low peak capacitor slowly charges upwards from the initial bias signal level to the lowest "tail" level. In this instance, the threshold voltage just after receiving the initial high amplitude pulse is set to a level midway between the high peak and the initial bias level. During the slow charging period, the threshold voltage increases slowly above that midpoint, which may be below or in the tail region of the diode's response characteristics. If the former, the comparator may not trigger on pulses received during this time period, and if the latter, the comparator produces pulses of substantially greater pulse width than that of the received pulses.
The distance between the LED and the photo diode typically may range from 1 cm to 1 meter. If the photo diode and LED are so close that they touch, the photo diode tends to generate a high amplitude current with a distorted pulse width. If the threshold voltage in the foregoing adaptive trigger circuit is set too far below the preamplifier's peak output, the comparator's output pulse width will also be much greater than that of the received light pulse. The foregoing problems cannot reliably be solved by ratioing the resistors to increase the threshold voltage above the midpoint. At minimum signal levels, such an adjustment would likely increase false triggering of the comparator output, and thereby decrease the receiver's noise immunity.
In view of the foregoing, it would be desirable to provide for use with a comparator an improved adaptive threshold circuit that has a first or low threshold voltage for triggering low input signal pulses of the type that might be generated by a photo diode, and that for higher input signal levels adapts the trigger level such that the photo diode's unique output current characteristics do not undesirably alter the triggering of the comparator.
It also would be desirable to provide for use with a comparator an improved adaptive threshold circuit for producing a variable threshold voltage such that the comparator correctly responds to the initial pulse.
It also would be desirable to provide for use with a comparator an improved adaptive threshold circuit that provides low differential-mode DC offset at the comparator inputs.
It also would be desirable to provide an adaptive threshold circuit that operates over a wide range of input signal frequencies and pulse amplitudes.
It also would be desirable to provide for a comparator an adaptive threshold circuit that adjusts for asymmetrical photo diode pulses such that the threshold is set in the "sharp cutoff region" following the falling edge of high amplitude photo diode pulses.
It also would be desirable to provide for a comparator an adaptive threshold circuit that adjusts for distorted photo diode pulses such as those that result when the LED and photo diode touch, such that the threshold voltage is set just below the preamplifier output peak voltage, to accurately reproduce the received pulse width.